I would view your Scarabs as an incredible force multiplier. Given the large amount of high volume strength 6 and 7 attacks in your army, being able to drop the AV of a Land Raider / Russ / Battlewagon / Predator by even 2 points would make those weapons
soooo powerful. Against lesser vehicles, you're probably wrecking one a turn with a couple of Tesla Destructors.
Being inexperienced with Necrons, I wonder if you might be better off with a pair of smaller Scarab squads, and a single large Wraith unit? That way, you could cripple, then kill, two vehicles a turn, and still have a real hammer unit to smash things with. If your opponent is running mostly infantry, then you could use the scarabs as tie-ups for shooty infantry, while your Destructors deal death to choppy infantry... and your Wraiths do their thing.
Like I said, I don't have any meaningful experience with 'crons, so that may not be viable.